When the opponent opens with 1NT, your choice to bid low is negated as he is already bidding last of level 1 and also showing 15-17 HCP. It leaves you on a back foot as any 2 level overcall requires you to have 10+ points. To counter opener, overcaller can take following actions using GIB convention card used by BBO robots (mostly, inspired by Standard American)

• 0-9 Points : just pass. Overcall is undesirable
• 10-14 Points but No Clear Trump Suit : Balanced hands are ideal for defence not offence. If you have a hand with no clear trump suit, DO NOT Overcall, just pass.
• 10-14 Points with Long Suit : go for CAPPELLETTE convention.
• Competing Hand with 15+ points :DOUBLE to show your partner a strong hand. Partner is free to pass and treat it as penalty double. However, with a 5+ suit he should bid.

Note : Double by passed hand is a special situation. It does not mean 15+ points. Reason-this hand had a chance to bid if he had such strong hand, opening bid was in order by him earlier which was passed, instead. Shows less than opening hand strength but interest in Major suit. A hand with 4+ both majors, may be.

• Vulnerability : Do consider unfavourable impact of Vulnerability especially where you’re vulnerable and opponent is not.

Cappellette is a defensive bidding convention particularly recommended for use against a 1NT. It allows overcaller to show a variety of two-suited hands that helps the partnership discover best FIT, quickly.

Cappelletti overcalls should be bid with 10-14 HCP, in unbalanced hands, using 2!c, 2!d, 2!h, 2!s or 2NT artificial alert bids.

• Overcall 2!c – When 6+cards suit or a strong 5+ cards suit with 8+ suit quality.
  • Advancer’s 2!d artificial relay bid directs overcaller to show the 6+/5+ strong suit at the lowest level. If the long suit is DIAMOND Cappellette bidder will PASS.

Note : intention here is to keep the winning bid at level 2.

• Overcall 2!d-When hand has both MAJORS with minimum 5-4 configuration. 

Advancer will bid his long MAJOR at lowest level.

• Overcall 2!h or 2!s-When bid MAJOR and a MINOR in 5-4 configuration (5 of MAJOR). 

• Overcall 2NT-When both MINORS with minimum 5-4 configuration. 
  • Advancer bids his long MINOR at lowest level

A Cappelletti overcall is most usually made from second seat but if second and third seat pass, then fourth seat can also intervene with Cappelletti