Is this for your office or home? If so I assume you have a pc running at all times. Go to www.no-ip.com sign up for a free DDNS account and download and install the client. Youll be able to use a DNS name that will automatically update to whatever your IP address is. For example you might use telephones.bounceme.net. Ive been using No-IP for 8 or 9 years now.

https://www.no-ip.com/