Though outbound connections to PSTN lines in UK and India will work fine, that may not be the best approach. For example, on Voxbeam (Premium route), calls to UK are only $0.0058/min. (landline) and $0.0515/min. (most mobiles). You can find similar rates at e.g. Easyvoip (lower quality but less hassle). It's likely that your mother-in-law's rates are higher (perhaps much higher, depending on her landline plan). Connecting via a VoIP provider would likely be better quality, lower latency, have faster set-up times, and wouldn't tie up Mom's line.
In the case of India, there have been laws prohibiting casual (unlicensed) interconnection with the PSTN. Those may no longer be in force, but you should check it out before setting up such a system. Rates to India (both landline and mobile) are less than $0.015/min. with many providers.