hi rf engineer,
the only soft solution I know is to use a stronger codec : have you tried AMR-FR with a codec around 6 or 7 kb/s ?
This should be more resistant in poor radio conditions. You will need to increase the radio link timeout ‘amr’ to fully benefit from it. (in most recent radio releases, there is a new feature “repeated SACCH”, that is useful for robustness of AMR against call drop)
Physical solution : add a cheap indoor repeater for this user, this will cost few hundred $.
1 outdoor antenna
2 feeders 1/2″
1 repeater (2-ways amplifier)
1 indoor antenna
It’s an easy upgrade as long as there are not too many frequencies to repeat. If the GSM band is very used, then you’ll have to invest in a band-selective or even channel selective repeater (more $$).