Good plan.
With regard to the mechanics of porting, you will be porting a land line number to another land line carrier (which might have been what your cable company meant). Land line ports follow a different and more complex process than simple mobile<--> ports. This is partly due to legacy FCC Local Number Portability (LNP) regulations, that were enacted in an attempt to prevent bad actors from stealing people's phone numbers (porting them away without the customer's permission). The procedure involves both the "losing" carrier (cable co.) and the "gaining" carrier (whichever carrier you select). You give the gaining carrier authority to act as your agent, (this is known as a LOA or Letter of Agency). The gaining carrier then requests to the losing carrier, that they unlock/release that number for porting out, and they provide some authentication information (typically your account number and service address and optionally a PIN). The losing carrier checks to confirm that the authentication info is valid, and, if so, they release the number and establish a "Firm Order Commitment" or FOC date. After that point, it is all up to the gaining carrier to take the number and re-route calls, effective on the negotiated FOC date/time.