IP Phone sometimes does not receive calls from my own mobile phone.

[willywonka]

I own the IP Phone W60P W56H W60B.Only one handset and one sip.I use it as a private phone.It receives incoming calls without a problem however, it is a hit and miss affair when I am out and want to phone the handset with my mobile phone.The mobile phone rings the home number but the Yealink phone is dead.No ring tone ,nothing at all, not even a missed call message.I can call my mobile phone with the Yealink phone without problem.It has received calls from other mobile phones and landlines without problem.I have noticed if I switch off the base and turn it back on the handset will ring when I place a call from my mobile phone.There have been times when I've tested the handset by placing a number of mobile phone calls to it, spaced a few minutes apart.The handset manages to receive 3 or 4 calls and then say, on the 5 attempt, the handset will not ring.I don't understand the intermittent nature of the phone being able to receive a call from my mobile in one moment and then, unable to do so in the next?
Any help would be be appreciated.The phone is critical because I'm a carer for my elderly mother.