I have found some PC's and laptops will require an external modem to make a solid connection, try some older posts for the brand and type. If you are dialing in, you may try connecting to an ext port direct to your modem, set it to auto line selection to intercom. dial string 76 for the internal partner modem. I have had better luck in this fashion than dialing in, but it is not perfect. I am not sure if you are trying to connect remote, or is you are on site.
You do not always get what you pay for, but you never get what you do not pay for.