Ongoing internet connection problems .

Could anyone help identify an ongoing problem that I’ve had with my ISP where the internet connection is cut when I receive or make a telephone call ?
I have a landline telephone , a Toshiba laptop , Verizon is my ISP , I’ve replaced the modem/router three times , the telephone cable and cable to router/modem , a filter and the splitter from the wall which splits the line to telephone and modem/router .
Tomorrow will be the sixth time the Verizon Technician will be out to see if he can , this time , identify the problem which has existed since December 2012 .
I should also mention that rarely do I get the three MBPS that Verizon sells and the speed is usually less than three MBPS and most of the time it’s less than one MBPS .
Furthermore , after a year of this my patience is close to exhaustion and just where do I lodge a formal complaint about the lackluster ‘service’ that Verizon has provided ?
Despite my sincerest , and polite , attempts to get Verizon to take care of these issues they’ve been , to date , miserable failures .
Any suggestions and recommendations will be much appreciated .
Thanks to those who answer .
=)