Is Capital Gain Tax payable on SIP shares?

Hi, My partner had shares through an employee SIP (a matched shares scheme). The scheme ran from 1996 to 2002 and then the company changed the scheme but the shares continued to be held there. Although she left the company in 2016, the shares continued to be held in the plan until this year when the company gave notice that the scheme would close. She was given the option of transferring the shares to a broker or simply selling them. She chose to sell them. Thinking she would have to pay Capital Gains Tax on the proceeds, she completed a CGT declaration and submitted it. However, now she's had chance to look into it more, she's not sure if the shares should have been subject to CGT, due to them being purchased through a SIP scheme. She's been trying to speak to someone on the phone at HMRC but just getting directed to Self Assessment and then a message saying the service is closed until September. Can you please help? Many thanks for your help Mike

UPDATE: We were able to get the answer after all - spoke to someone on the phone and also had the answer given in another part of the forum. No need to reply. Many thanks Mike