It catches network when their is strong signal but in areas that are not having strong signals it says no network coverage

Keep in mind, poor reception is not something that your service provider will fix just because you want them to. It must be justifiably cost effective for them to put up new towers, which will ultimately be criticized by cities, home owners associations, etc. Most users think there is nothing they can do to improve their cell phone reception by themselves. This is most certainly untrue, and the following will explain what you can do to ensure great mobile coverage without waiting for a new tower to magically appear.
edit Steps

1. 1Battery. Your cell phone uses a lot more power when connecting a call than when it is on standby. Often, your battery can be strong enough to attempt a call, but not strong enough to find a signal. If you find you are having signal problems, try to keep your battery charged to 2 bars or higher. Ads by Google
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2. 2Landscape. Buildings and other large structures are very unfriendly to cell phone signal. Rather than making calls from deep inside buildings try moving outside or to a window to place your call. If you are having reception problems on the street try walking to the nearest intersection as they typically have better coverage. Cellular band radio waves do not effectively penetrate earth: if you are underground you will likely receive no signal.
3. 3Install a cellular repeater. If you are having cell problems in one location, such as your home or office, then try installing a cellular repeater. Cell phone repeaters pick up low cell signal with an antenna, boost the signal and broadcast it over the coverage area. They typically need at least 2 bars of signal where the antenna is placed (usually outside or on the roof) but can substantially improve cell reception, as well as battery life and data download speeds. Some repeaters might need technical knowledge such as the frequency of your carrier, and only work for one service provider. For a less technical approach that improves reception on all carriers, use a dual-band cell phone repeater.
4. 4Upgrade your antenna. A few cell phone manufacturers make a "Hi-gain" antenna for their handsets, which may be changed in-store or by the user at home. Although these won’t improve signal as much (or at all) as a repeater these antennas are relatively inexpensive and you are not confined to one location.
5. 5Change networks. Most networks operate independently of one another, using their own frequencies and constructing their own cell phone towers. Chances are if the signal is bad with one network you can improve by switching. Most cellular networks these days allow you to transfer your phone number when you change provider.
6. 6Hold your phone correctly. Mobile phone antennas are designed to project a signal outward, perpendicular to the long axis of the antenna. As such, mobile phones seek signals in a donut-esque shape around the antenna. Normally, when a mobile is held upright, this is not a problem. However, if you are holding your phone in a strange way, such as on its side or upside down, you will hinder the operation of the antenna. Hold your phone upright to guarantee that your phone can "see" your carrier signal.
7. 7Host a cell site. This may take time, but where cell phone reception is inadequate property owners can host small cell sites on their properties for major wireless carriers. 3rd parties with Wireless Revenue Programs allow you to register your property to be eligible. Then when there is carrier interest in the area you'll be on the short list of places they choose from and will have optimal coverage. They may even pay your phone bill.